OUTLINES 


OF  THE  HALF-COURSE  IN 


Natural  History 


WITH  REFERENCES  TO 


DANA’S  MANUAL  OF  GEOLOGY, 

AND  NOTES. 


COPYRIGHT,  i88s- 


(S^ambribgc,  Pass.: 

Wm.  H.  Wheeler,  Publisher. 
1885. 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/outlinesofhalfcoOOunse 


OUTLINES  OF  NATURAL  HISTORY  4. 


INTRODUCTORY. 


The  Earth. 

How  REGARDED.  Dana^  ff,  1-2. 

Earth  is  to  be  treated  as  a mechanism  which  has  a history  ; 
as  a theatre  (i)  of  the  application  of  force,  (2)  of  the 
development  of  life.  It  is  as  truly  as  is  the  steam-engine, 
a mechanism  which  is  applying  solar  force. 

Kepler’s  idea  of  the  earth  as  a living,  breathing  crea- 
ture is  nearer  right,  than  that  of  an  utterly  inanimate 
mass. 

Relation  to  Universe.  Dana,  ;p.  j. 

Three  proofs  of  Earth’s  kinship  to  pkwirts  : — 

(1)  Telescopic — spherical  form,  continents  and  volcanos. 

(2)  Meteoric — elements  and  laws  of  crystallography 
same. 

(3)  Spectroscopic — elements  of  sun,  etc.,  same  as  those 
of  earth. 

Aim  of  Geology.  Dana,ff.  4.-6. 

I.  Physiographic  Geology. 

Earth’s  Contour  and  Surface  Divisions. 

Its  form.  Dana,  f.  p. 

Flattening  of  earth  is  correlated  with  its  rotation,  being 
just  what  would  result  from  earth’s  present  angular  veloc- 
ity in  a liquid  globe  of  same  size  and  density.  This  gives 
proof  of  the  present  or  former  molten  state  of  the  earth. 


OUTLINES  OF  NATURAL  HISTORY  4. 


4 


Land  and  Water.  Dana^j>^,  lo,  ij. 

From  the  cooling  and  development  of  an  original  nebu- 
lous mass,  there  results  a solid  surrounded  by  two  oceans, 
of  water  and  air,  respectively.  Much  depends  on  the 
interaction  of  these  two  oceans,  one  partially,  the  other 
entirely  covering  the  earth’s  surface.  We  see  on  surface 
of  globe  a great  land-mass  about  No.  Pole  from  which 
three  triangles  project  southward  into  mass  of  water 
about  So.  Pole.  Note  that  oceans  keep  practically  same 
level,  while  lands  rise  and  fall.  Two  great  geologic 
forces  are  constantly  at  work  on  land,  those  above  and 
at  sea-level  being  acted  on  by  erosion^  the  destructive 
force,  and  those  below  sea-level  being  subjected  to  depo- 
sition^ the  constructive  force. 

Depth  of  Oceans.  Dana^  ff.  //,  12. 

Note  that  this  is  on  the  average  much  greater  than  the 
elevation  of  land-masses. 

Surface  Reliefs.  Dana^  fp,  15-23. 

Substitute  for  definition  of  mountain as  follows  : — A 
mountain  is  a ridge  of  the  earth’s  surface  in  which  at  least 
a part  of  the  relief  is  due  to  the  folding  of  strata. 

A hill  is  a relief  caused  by  the  cutting  away  of  parts  of 
strata  by  erosive  action,  leaving  remainder  as  an  elevation. 

A volcanic  cone  is  an  elevation  produced  b}^  a deposit 
of  substance  ejected  from  a crater. 

System  in  Earth’s  Reliefs  and  Feature-lines.  Dana^ 
fp.  23-38. 

Dana  has  been  chief  student  of  this  subject  and  has  given 
it  undue  prominence  here.  Read  especially  “Law  of 
the  System,”  p.  23,  and  “Recapitulation,”  p.  37.  Great 
exception  to  general  N.  W.  and  N.  E.  trends  is  the  Indo- 
European  mountain  system  which  trends  E.  and  W. 

Atmospheric  Currents.  Dana^p.pj.3. 

These  give  us  key  to  oceanic  currents  and  are  therefore 
taken  up  first.  If  we  had  no  atmosphere  of  air,  we 
should  have  one  of  steam  as  a result  of  the  sun’s  heat 


PHYSIOGRAPHIC  GEOLOGY. 


s 


which  is  sufficient  to  melt  8000  cu.  mi.  of  ice,  daily. 
Were  there  no  transportation  of  heat,  the  temperature 
of  the  tropics  would  be  perhaps  150°  and  that  of  the  poles 
— 50°  F.  This  would  leave  but  a narrow  belt  where  life 
could  exist  and  it  would  probably  be  kept  off  there  by 
furious  winds.  But  since  air  is  easily  penetrated  by  direct 
heat  and  holds  that  which  has  entered  it,  lower  air  at 
Equator  becomes  very  much  warmed,  and  rises,  causing 
an  inflow  of  cooler  air  from  poleward  regions,  to  balance 
which  an  outflow  towards  poles  takes  place  in  upper  air. 
This  would  give  north  and  south  winds  toward  the  Equa- 
tor in  the  tropics.  But  as  earth’s  rotation  causes  an  effect 
like  that  of  a turn-table,  throwing  moving  bodies  on  its 
surface  to  the  right  of  a direct  course,  these  currents  of 
air  are  thrown  to  right,  and  N.  E.  and  S.  E.  wind  pro- 
duced, known  as  the  Trades.  But  air  is  a very  poor 
conductor  of  heat  and  acts  to  equalize  temperature  only 
in  a secondary  way. 

Oceanic  Currents.  Dana^  p.  j8. 

Trade  winds  blowing  steadily  over  the  oceans  within  the 
tropics  toward  N.  W.  and  S.  W.  cause,  by  their  con- 
stant brushing  over  the  surface,  strong  currents  in  the 
same  directions,  which  meet  near  the  equator  and  flow 
out  on  a westerly  course.  If  earth’s  surface  were  wholly 
water  this  current  would  simply  follow  the  equator,  but 
being  broken  up  by  continents,  it  is  turned  N.  and  S. 
and  carries  into  Temperate  and  Polar  regions  vast  quan- 
tities of  heat. 

Thus  in  Atlantic  the  equatorial  current  splits  on  C.  St. 
Roque,  and  losing  influence  of  trades,  a great  current  is 
carried  by  its  own  inertia  up  the  western  shore  and 
across  the  ocean  to  its  northeastern  shore,  and  on  in  part, 
into  the  Arctic  Zone.  A part  of  this  great  "Gulf  Stream” 
returns  along  the  coast  of  Europe,  but  much  the  greater 
part  returns  along  the  bottom  of  the  ocean,  as  a slow,  cold 
current.  Dr.  Croll  has  shown  that  the  Arctic  Zone  gets 


OUTLINES  OF  NATURAL  HISTORY  4. 


6 


more  from  Gulf  Stream  than  from  sun  directly,  and 
many  parts  of  the  earth  are  made  habitable  by  warm  cur- 
rents. Since  Cape  St.  Roque  is  some  distance  south  of 
equator,  the  No.  Atlantic  O.  gets  more  than  its  share 
of  the  equatorial  current.  To  this  fact  is  due  the  climate 
of  England  and  her  history.  Great  Japan  current  of  the 
Pacific  does  not  send  any  branch  into  Arctic  O.  If 
Alaska  were  below  the  surface  this  would  happen  ; and 
this  may  perhaps  explain  the  former  mild  climate  of 
Greenland. 

Gulf  Stream  had  long  been  known  in  a general  way, 
but  was  first  definitely  studied  by  Benj.  Franklin,  though 
he  never  clearly  published  his  results.  First  careful  study 
of  ocean  currents  was  by  Maury,  who  plotted  them  from 
study  of  a large  series  of  ship’s  logs.  He  believed 
them  caused  by  greater  height  (17  ft.)  of  water  at  equa- 
tor than  at  poles,  due  to  its  expansion  from  greater  tem- 
perature at  equator.  Dr.  Croll  has  shown  that  this 
gradient  is  incompetent  to  produce  the  known  results. 

Waves  are  also  caused  by  winds,  but  are  only  undulations 
not  implying  motion  of  translation.  Their  effect  is  to 
strike  a blow. 

Climate.  Dana^f.4.^, 

Land  climate  is  result  of  interrelations  of  water  and 
heat,  and  furnishes  greatest  extremes.  Sea  climate  is 
much  less  variable.  As  an  example  of  effect  of  absence 
of  water,  moon’s  surface  is  dead,  without  atmosphere  or 
life.  Only  changes  there  are  due  to  expansion  and  con- 
traction caused  by  enormous  daily  extremes  of  temper- 
ature (perhaps  1000°  to  — 200°  F.) 

At  the  critical  point  of  32°  F.  the  influence  of  water 
is  wholly  changed.  Above,  it  is  a liquid,  readily  absorb- 
ed and  essential  to  life  ; below,  it  is  a ver}^  slowly  moving 
solid,  fatal  to  life. 

Distribution  of  Fertile  and  Sterile  Lands.  Dana, 
f.  44. 


LITFIOLOGICAL  GEOLOGY. 


7 


II.  Lithological  Geology. 

Rocks.  Dana^  -pf.  ^7,  62. 

This  term  is  used  to  include  all  the  materials  of  the  earth’s 
mass,  excepting  imbedded  ice.  Rocks  give  us  the  chief 
points  in  the  study  of  the  earth’s  physical  history,  while 
their  contained  fossils  furnish  much  of  the  history  of  life. 

They  may  be  divided  into  two  great  groups,  according 
to  their  origin. 

Mineral  Rocks  are  those  which  have  originated  solely  from 
physical  and  mechanical  causes,  including  by  far  the 
greater  mass  of  rocks,  the  sandstones,  granites,  etc.  Ac- 
cording to  structure  they  may  be  divided  into  two  groups  : 

Fragmental  rocks,  formed  of  broken  fragments  of 
various  rocks,  are  produced  wholly  by  action  of  celestial 
forces  working  through  water.  These  include  sand- 
stones, shales,  etc.  As  soon  as  laid  down  these  rocks 
become  subject  to  action  of  terrestrial  and  celestial  forces, 
for  as  new  strata  are  laid  down  over  them,  the  effect  is  to 
raise  the  temperature  in  lower  strata  in  the  same  way  as 
if  a series  of  blankets  were  piled  up  over  them,  each  new 
blanket  serving  to  raise  the  temperature  at  any  given 
point.  This  blanketing  thus  causes  the  lines  of  equal 
earth  temperature  (isogeothermal  lines)  to  rise,  and  as  a 
result  of  increased  temperature  and  pressure  the  fragmen- 
tal rocks  become  slowly  changed  to  the. second  or  crystal- 
line form  which  is  the  state  of  rest  of  rock  material. 

Crystalline  rocks  include  granite  quartz,  schists,  etc. 
The  effect  of  erosion  on  these  rocks  is  to  carry  them  back 
to  the  fragmental  form,  thus  completing  the  cycle. 

Organic  Rocks  owe  their  origin  to  physical  and  organic 
causes  combined,  and  consist  almost  wholly  of  compacted 
remains  of  organisms.  They  include  the  limestones. 

Elements  in  Rocks.  Dana,  f.  ^8. 

Add  to  those  enumerated  by  Dana,  phosphorus,  the  re- 
lation of  which  to  life  will  be  noticed  later. 


OUTLINES  OF  NATURAL  HISTORY  4. 


8 


? 


Distinctions  in  Study  of  Rocks.  Dana^f.  6j. 

Note  that  specific  gravity  of  rock  is  necessarily  greater 
than  that  of  water ; otherwise  the  earth’s  machinery  must 
be  stopped.  Note  carefully  descriptive  terms,  p.  66. 

Conditions  and  Structure  of  Rock  Masses. 

Stratified  Condition.  Dana,  f,  /p. 

Stratified  rocks  are  those  arranged  in  layers,  in  whose  de- 
position there  has  been  an  element  of  gravity.  They 
may  be  formed  in  three  ways  : (i)  by  falling  of  matter 
through  water  to  bottom  ; (2)  by  falling  of  matter  through 
air,  as  from  a volcano;  (3)  by  lava  flow  from  fissure  or 
crater. 

A layer  (p.  81)  is  the  structural  unit  in  stratification  ; 
it  indicates  the  the  continuation  of  certain  forces  for  a 
time,  followed  by  a change. 

A formation  consists  of  several  or  many  strata  of  many 
layers  each.  It  is  less  clearly  marked  than  a stratum, 
which  is  not  so  readily  distinguishable  as  a layer. 

Structure  of  Layers.  Dana,  p,  82. 

Of  structures  originating  in  the  act  of  deposition,  the 
laminated  and  shaly,  separated  by  Dana,  are  practically 
the  same.  Compound  structure  is  due  to  the  action  of 
winds,  waves,  or  currents.  Strata  formed  by  the  wind 
are  of  uniform,  fine  material ; those  formed  by  water  may 
be  of  various  materials.  Strata  formed  by  regularly 
flowing  currents  are  of  an  uniform  inclination,  while  those 
formed  by  tide  currents  or  waves  may  be  very  uneven  and 
broken. 

Ripple  marks  {p.  84.)  indicate  the  presence,  at  time  of 
their  formation,  of  shallow  water  and  a current,  but  not 
necessarily  of  a beach,  since  they  are  known  to  be  formed 
in  water  200  ft.  deep.  They  are  like  marks  formed  by 
wind  on  sand  or  snow.  How  this  action  starts  and  what 
determines  height  and  distance  of  ridges  is  unknown, 
though  it  may  be  a rolling  motion  of  air  or  water.  Both 
measurements  are  greater  in  water  than  in  air. 


LITHOLOGICAL  GEOLOGY. 


9 


Mud-Cracks  are  such  as  are  commonly  formed  under  a 
hot  sun  on  muddy  shores,  and  are  often  and  easily  fos- 
silized by  being  filled  with  blowing  sand  or  some  other 
substance,  and  then  buried  under  fresh  mud. 

Rain  Drof  prints  are  well  preserved  on  some  old 
beaches ; from  them  we  can  often  tell  the  direction  of  the 
wind  during  the  shower,  by  the  splash  on  the  leeward 
side  of  the  print. 

Of  structures  of  origin  subsequent  to  la3fing  down  of 
strata  : — the  concretionary  structure  ( -p.  8^)  somewhat 
resembles  the  crystalline,  but  differs  in  that  crystallization 
takes  place  in  accordance  with  mineral  species,  at  high 
temperatures,  and  results  in  definite  geometrical  masses, 
while  concretion  takes  place  more  with  reference  to  sub- 
stance of  matter  deposited,  at  lower  temperatures,  and  re- 
sults in  amorphous  masses.  Concretions  often  begin  about 
an  organic  nucleus.  Sometimes  they  are  hollow  or  with 
interior  filled  with  crystals.  They  often  simulate  animal 
forms,  as  in  case  of  a form  found  in  very  old  limestones 
in  Canada  and  named  Eozoon  Canadense^  whose  true 
nature  was  for  20  years  a subject  of  much  dispute,  many 
believing  it  to  be  the  fossil  remains  of  a very  simple 
animal.  Concretion  and  crystallization  are  the  two  great 
causes  of  metamorphosis  in  rocks,  and  are  greatl}^  aided 
by  the  presence  and  circulation  through  the  rocks  of 
waters  having  mineral  matters  in  solution.  A similar 
action  is  the  taking  on,  by  the  constituents  of  rocks,  of 
more  of  their  own  material  from  solutions,  thus  more 
firmly  cementing  the  rock  together.  This  is  called  in- 
terstitial metamorphosis. 

All  but  the  newest  rocks  are  penetrated  by  breakage - 
lines  (^.  88^  usually  in  different  directions,  and  irregu- 
larly placed. 

These  various  planes  cut  up  the  rock  and  facilitate 
quarrying,  where  not  so  numerous  as  to  render  the  rock 
worthless.  The  cause  of  formation  of  these  joint- 


OUTLINES  OF  NATURAL  HISTORY  4. 


lO 


-planes^^  is  probably  successive  enormous  pressures  to 
which  the  rock  has  been  subjected,  each  set  of  joint-planes 
being  the  index  of  a pressure  at  right  angles  to  its  direc- 
tion. 

Slaty  cleavage  {p.  8g)  is  also  caused  by  pressure,  but 
differs  from  jointed  structure  in  having  its  planes  close 
together  and  in  a single  direction  only.  The  rocks  in 
which  this  structure  occurs  were  formerly  clayey  mud 
which  contains  much  mica  in  tiny  particles.  Pressure  on 
this  mud  would  bring  the  bits  of  mica  more  nearly  par- 
allel and  start  fracture-lines. 

Movements  of  rocks  on  each  other  cause  groovings, 
polishing,  etc. 

Positions  of  Strata.  Dana,  p,  gi. 

Natural  position  of  strata  is  horizontal,  the  tendency 
being  to  level  irregularities,  and,  on  an  inclined  surface, 
to  form  an  incline  of  less  angle.  River-deltas,  however, 
form  beds  which,  at  their  further  ends,  have  steep  slopes. 

Dislocations  of  strata  caused  by  powerful  forces  are 
common,  and  of  several  forms.  The  simplest  is  the  fold, 
which  may  be  simple  or  modified  by  subsequent  greater 
folds,  or  may  be  collapsed,  that  is,  pressed  together  till 
there  is  no  longer  resistance  to  pressure  and  its  sides 
meet,  the  whole  fold  falling  over  sidewise.  The  line  of 
direction  of  the  fold  is  called  its  axis.  An  upfold,  with 
sides  sloping  away  from  a ridge  is  called  an  anti- 
clinal; a downfold  with  sides  sloping  into  a trough,  a 
synclinal.  The  compass  course  of  the  axis  of  a fold  is  its 
strike,  while  the  slope  of  its  sides  is  the  dip.  These 
folds,  etc.,  are  negative  results  of  the  cooling  of  the 
earth’s  interior  and  consequent  shrinkage  and  packing 
of  the  crust  to  accommodate  itself  to  a smaller  nucleus. 

Faults  are  usually  results  of  similar  forces  to  those 
causing  folds,  and  occur  where  rocks  from  their  structure 
or  from  being  thick-bedded,  do  not  readily  bend  under 
pressure,  but  rend  apart.  Faults  very  commonly  follow 


I 


LITHOLOGICAL  GEOLOGY. 


1 1 

joint  planes,  and  may  be  of  any  size.  In  most  cases 
there  has  been,  in  addition  to  rending,  a slipping  of  rocks 
on  opposite  sides  of  the  fault  upon  each  other  and  this 
sometimes  amounts  to  thousands  of  feet,  bringing  strata  of 
very  different  age  on  to  same  level.  Fault-planes  may  be 
located,  like  folds,  by  dip  and  strike.  The  portion  of 
rock  faulted  out  of  place  is  usually  wedge-shaped,  and 
in  a large  majority  of  cases  the  strata  have  been  pushed 
in  the  direction  indicated  by  the  point  of  the  wedge. 
Since  a joint-plane  is  commonly  a warped  surface  the 
result  of  slipping  of  rocks  is  often  to  form  open  spaces 
where  sides  fail  to  fit  closely,  and  many  mineral  veins 
have  originated  in  this  way.  Another  form  of  fault  is 
caused  by  splitting  of  rock  mass  from  shrinkage,  forming 
“Gash-cracks.*’ 

Even  the  hard  rocks  are  fluid  to  sufficiently  great 
pressure  and  may  be  shown  to  have  been  compressed 
so  as  to  be  increased  many  times  in  length,  by  distorted 
fossils  found  in  them.  These  can  be  recognized  when 
extended  to  three  or  four  times  their  length,  but  beyond 
this  become  mere  blurs.  One  of  the  hardest  tasks  of 
the  Palaeontologist  is  the  reconstruction  of  distorted 
fossils. 

Great  sources  of  error  in  study  and  practical  estimation 
of  strata  are  folding  and  faulting,  by  which  a single 
stratum  may  be  brought  to  surface  at  several  points  and 
so  made  to  seem  like  so  many  strata.  The  geologist 
avoids  error  of  this  sort  by  carefully  noting  the  thickness 
of  seemingly  different  strata  as  well  as  the  character  and 
thickness  of  beds  immediately  above  and  below  them. 
Similarity  in  these  data  indicates  pretty  surely  the  iden- 
tity of  what  seem  to  be  distinct  beds. 

Effects  of  denudation  and  conformable  and  unconfor- 
mable  strata  are  sufficiently  treated  in  text  g6,  loo). 

Arrangement  of  Strata.  Dana,  f.  loi. 

Means  of  determining  the  order  of  arrangement  of  strata 


OUTLINES  OF  NATURAL  HISTORY  4. 


12 


are  several,  but  the  most  important  is  the  stud}’  of  their 
fossils.  It  was  suggested  200  years  ago  by  Dr.  Hooke 
that  it  might  be  possible  to  "raise  a chronology”  from  the 
study  of  fossils,  but  nothing  was  done  until  the  begin- 
ning of  the  present  century,  since  which  time  it  has  been 
shown  by  careful  study  of  fossils  that  there  have  been 
many  nearly  complete  changes  in  the  organic  life  of  the 
earth’s,  surface.  This  was  long  believed  to  be  due  to  the 
sudden  extinction  of  life  and  the  subsequent  creation  of 
new  forms ; but  it  is  now  believed  that  at  various  times 
new  forms  have  arisen  from  existing  forms  by  rather 
rapid  changes  and  modifications,  though  we  cannot  say 
what  forces  have  determined  these  changes.  The  study 
of  these  organic  remains  enables  us  to  identify  layers  or 
beds  of  the  same  approximate  age,  to  study  ancient 
climates  from  the  distribution  and  limits  of  species,  and 
to  study  the  currents  of  the  ancient  seas,  and  from  them 
continental  lines,  chiefly  from  remains  of  reef-corals 
which  require  food-bearing  currents  for  their  growth. 

From  these  studies,  materials  are  accumulating  for  a 
complete  history  of  the  ancient  conditions  and  changes 
of  the  earth,  since  organic  life,  on  account  of  its  delicacy 
and  impressibility,  preserves  for  us  in  its  remains,  records 
of  many  important  facts  of  which  the  rocks  give  no  hint. 
But  the  old  idea  that  similarity  of  species  of  fossils  in 
two  layers  indicates  identity  of  age  must  not  be  too  much 
trusted  since  a given  region  of  the  globe  may  have  had 
at  one  time  a very  similar  fauna  to  that  of  another 
region  at  a very  different  geologic  period.  For  instance, 
as  we  go  away  from  Europe  we  find  life  differing  more 
and  more  from  the  life  of  that  continent,  and  find  that  the 
present  fauna  of  N.  A.  much  more  closely  resembles  the 
Middle  Tertiary  fauna  of  Europe  than  that  of  to-day, 
while  the  present  Australian  fauna  is  very  like  that  of 
Europe  in  the  Jurassic  period. 

It  should  be  noted  that  fossils  “of  the  same  age”  may 


LITHOLOGICAL  GEOLOGY. 


13 


represent  organisms  which  lived  thousands  of  years 
apart,  but  under  similar  terrestrial  conditions,  that  is,  in 
the  same  geologic  period.  Fossils  of  the  same  age  or 
geologic  period  are  said  to  belong  to  the  same  ^'‘horizony 

Unstratified  Rocks.  Dana^  f,  loj. 

These  rocks  may  be  divided  into  three  classes;  (i) 
lavas  which  have  flowed  from  craters  ; these  sometimes 
present  a stratified  appearance  from  the  superposition  of 
successive  flows,  and  on  the  other  hand  some  bedded 
rocks  have  been  so  metamorphosed  by  heat  as  to  resem- 
ble lavas;  (2)  lavas  which  have  flowed  between  bound- 
ing walls  of  rifts,  forming  dikes  ; and  (3)  deposits  from 
solutions  in  water,  such  as  those  of  mineral  matter  in 
veins.  To  these  ma}^  be  added  for  convenience,  glacial 
drift  which  owes  its  origin  to  cold,  while  those  properly 
belonging  here  are  the  result  of  heat-action. 

Dikes  and  Veins.  Dana^  -pf.  108-1  ij. 

Both  these  formations  originate  as  rifts  or  cracks  in  the 
rock,  caused  by  faulting  or  otherwise,  and  usually  follow- 
ing  joint-planes  which  become  filled  by  diflerent  mineral 
substance. 

Dikes  are  formed  of  full  width  from  the  first.  When 
a series  of  beds,  with  a rift,  becomes  greatly  heated 
by  the  blanketing  eflect  of  strata  lying  above,  and  one 
bed,  being  more  fusible  than  the  rest,  becomes  melted,  the 
effect  is  to  at  once  force  this  molten  material  upward  into 
the  rift,  with  or  without  volcanic  action,  filling  the  rift  and 
forming  a dike,  which  usually  does  not  extend  to  the  sur- 
face and  is  not  exposed  until  the  overlying  strata  have 
been  planed  away,  ages  later.  Dikes  are  usually  verti- 
cal and  often  very  irregular ; they  frequently  intersect, 
when  their  relative  ages  can  be  made  out. 

Fissures  not  filled  by  dike-material  are  sure  to  be  sooner 
or  later,  by  slow  process,  filled  with  deposits  from  per- 
colating water,  forming  veins.  These  fissures  may  be 
formed  at  first  of  their  full  width,  but  oftener  they  are 


OUTLINES  OF  NATURAL  HISTORY  4.  14 

gradually  spread  by  the  force  of  deposition  and  crystalli- 
zation of  substances  within ; this  is  called  interstitial 
growth.  All  substances  are  found  in  sea-water  in  some 
quantity,  it  is  supposed.  These  are  taken  up  by  plants 
and  animals,  as  for  instance,  lime  salts  by  corals,  mol- 
lusks  and  others ; soda  and  potash  b}^  sea-weeds ; and 
nearly  all  elements  in  minute  quantity  by  various  forms. 
When  these  plants  and  animals  die  and  decompose, 
these  elements  become  a part  of  the  rocks.  This  may 
be  termed  the  first  concentration  of  the  elements.  When 
deeply  buried  in  the  rocks,  where  the  water  has  a solvent 
power  equal  to  that  of  the  strongest  acids,  due  to  its  great 
temperature  and  pressure  and  large  proportion  of  CO'S 
these  substances  are  dissolved  and  the  percolating  water 
becomes  charged  with  them.  Here  we  have  the  second 
concentration.  If  now  we  have  a series  of  beds  with  one 
or  more  rifts  or  cracks  running  through  them,  we  shall 
find  this  saturated  water  working  downward  through  the 
permeable  strata,  which  are  bordered  by  impervious  ones, 
until  it  reaches  the  rift,  through  which  it  will  rise  rapidly, 
depositing  on  the  walls  the  substances  held  in  solution,  in 
the  order  of  least  solubility,  and  so  forming  veins  of 
metals  or  other  substances.  The  forces  which  cause  con- 
stant percolation  of  water  through  the  rocks  are:  (i) 
Gravitation  ; (2)  Osmosis  ; and  (3)  its  tendency  to.pene- 
trate  towards  a centre  of  heat. 


DYNAMICAL  GEOLOGY. 

{Dana,  Part  IV,  j>.  605.') 

Energy  from  the  celestial  spaces  as  applied  to  the  earth’s 
surface  may  be  divided  into  two  classes  : — the  general- 
ized, physical  forms,  the  winds,  currents,  etc.  ; and  the 
localized  form,  seen  in  organic  life. 


DYNAMICAL  GEOLOGY. 


15 


Life.  Dana^  f.  606. 

An  organic  form  may  be  defined  as  a form  of  matter 
which  has  the  power  of  taking  into  itself  matter  from 
without,  and  of  utilizing  its  material  and  force.  The 
work  of  earth-building  depends  on  the  interaction  of  or- 
ganic and  inorganic  forces.  The  tendency  has  been  to 
belittle  the  influence  of  organic  life  in  this  matter,  but  it 
is  true  that  hardly  any  existing  rock  would  be  in  just  ils 
present  condition,  but  for  this  influence.  In  crossing 
North  America  we  are  for  more  than  half  the  distance  on 
rocks  which  are  largely  the  result  of  organic  life,  and  it 
may  even  be  said  that  our  continent  owes  its  present  form 
to  this  really  powerful  agent. 

Its  Protective  Effects.  Dana^f.  6oy. 

The  most  important  protective  effect  of  vegetation  is  to 
protect  soil  and  roots  from  the  washing  effect  of  rains. 
A ploughed  field  loses  much  of  its  loose  surface-soil  with 
every  rainfall.  As  the  vegetation  depends  for  its  nour- 
ishment largely  on  the  soil,  so  it  protects  itself  by  hold- 
ing the  soil,  and  a given  piece  of  ground  depends  for  its 
soil  on  its  vegetation.  Thus  the  two  are  interdependent. 
Sea-weeds  protect  the  rocks  on  which  they  grow  by  pre- 
venting the  water  left  in  the  crevices  from  freezing  at 
low  tide  and  rending  the  rocks.  Our  salt  marshes  of  the 
New  England  coast  are  not  merely  protected,  but  have 
been  won  from  the  sea  by  maritime  plants,  chiefly 
grasses,  rushes,  and  sedges. 

Its  Transporting  Effects.  Dana,  p.  Soy. 

Its  Destructive  Effects.  Dana,  p.  6oy. 

These  are  better  called  Metaniorphic  effects  of  life, 
in  harmony  with  the  modern  idea  of  our  world,  that 
of  change,  rather  than  of  destruction  and  death. 

Decay  effectually  attacks  only  exposed  surfaces  of  rocks. 
Roots  in  their  growth  split  rocks  and  thus  expose  more 
surface  for  erosive  action.  A cubic  foot  of  rock  broken 
up  into  cubic  inches  has  more  than  ten  times  as  much 


OUTLINES  OF  NATURx\L  HISTORY  4.  1 6 

surface  as  if  in  a single  cubical  block.  Thus  plants 
help  to  make  the  soil  they  need.  If  the  Colorado  canons 
were  grown  with  vegetation,  their  vertical  walls  and 
sharp  angles  would  be  lost,  from  the  rounding  and 
eroding  action  due  to  the  leverage  of  plant-growth. 

Earthworms  do  a great  work  in  changing  the  surface  of 
the  ground.  Mr.  Darwin  has  shown  that  they  average 
40,000  to  each  acre  of  arable  soil ; and  these  pass  through 
their  digestive  systems  for  the  sake  of  the  organic  matter 
it  may  contain,  man}^  tons  of  earth  per  year  which  is 
left  at  the  surface  of  the  ground,  so  that  thus  the  surface 
is  often  complete!}^  changed.  Thus,  also,  stones  and 
small  objects  soon  become  buried  and  Mr.  D.  believed 
that  even  some  of  the  rural  churches  of  England  had 
been  undermined  and  caused  to  sink  noticeably  by  earth- 
worms. A similar  action  is  going  on,  by  worms,  on  the 
sea-floor.  Ants  also  do  a similar,  but  less  extensive  work. 

After  life  is  extinct,  many  organic  forms  have  much 
effect.  Many  growing  trees  shed  during  their  lives 
woody  matter  of  twenty  times  their  own  bulk,  yet  there 
remain  at  the  base,  but  a thin  layer  of  vegetable  mould, 
whose  constituents  are  constantly  being  absorbed  by  row- 
ing plants,  while  the  great  bulk  of  the  constituents  of  the 
original  wood  has  passed  into  the  air  and  been  more  or 
less  absorbed  by  plants,  in  the  gaseous  form.  Thus  the 
dust  to  which  we  return  is  but  a transient  passing  stage  be- 
tween life  and  life. 

Its  Contribution  to  Rock-Formations.  Dana^  -pf.  608- 
626. 

These  take  place  chiefly  under  water  where  decomposi- 
tion is  slow  and  imperfect.  Here  plant  and  animal 
remains  become  enclosed  in  the  rocks  as  stored-up 
carbon,  in  form  of  coal,  limestone,  etc. 

On  land,  one  of  the  chief  forms  of  deposit  is  the  peat- 
bog. These  are  formed  in  small  ponds  which  become 
gradually  filled  up  by  the  growth  of  peat  mosses. 


DYNAMICAL  GEOLOGY. 


17 


Starting  from  the  edge  of  the  pond,  these  mosses,  chiefly 
of  the  genus  Sfhagniim^  grow  out  slowly  into  the  pond, 
growing  upward  and  dying  below,  and  forming  a tangled, 
closely  matted  mass,  which  holds  water  like  a sponge. 
This  water  admits  of  but  very  slow  decay  of  the  dead 
moss,  which  gradually  falls  to  the  bottom  and  forms  a 
layer  of  black  mould,  between  which  and  the  sheet  of 
moss  is  a layer  of  water.  But  the  growth  of  the  moss 
increases  the  thickness  and  weight  of  the  sheet,  and 
Anally  it  reaches  the  bottom,  filling  the  basin.  As  the 
moss  is  very  yielding,  the  growth  always  forms  a veiy 
treacherous  bog.  In  some  bogs  in  Northern  N.  A.  the 
Sphagnum  may  grow  up  a slope  for  a few  feet,  but  in 
Europe  are  bogs  which  have  spread  hundreds  of  feet 
from  the  original  edge  of  the  pond.  These  are  called 
clhnhing  bogs.  They  sometimes  become  so  filled  with 
water  in  heavy  rains  as  to  burst  and  flood  the  surrounding 
country  with  black  mud. 

A peat-bog  must  have  one  of  two  fates.  It  may  be 
quickly  lowered  beneath  the  sea  and  then  buried  under 
rock  strata,  to  form  a coal  bed  under  influence  of  heat 
and  pressure.  If  slowly  sunk,  it  will  be  destroyed  by 
wave-action.  Or  it  may  remain  above  the  sea-level,  and 
be  finally  drained  by  cutting  down  of  surrounding  land. 
Then  the  dried  peat  will  decay  and  return  its  stored  carbon 
to  the  air.  As  temperature  of  tropics  causes  more  rapid 
decay,  no  important  bogs  or  coal  deposits  are  found  there. 
Peat  bogs  and  coal-deposits  are  often  underlaid  by  thin 
strata  of  so-called  ^Hfifusorial  earth which  consists  of 
remains  of  microscopic  plants  and  animals  which  lived  in 
the  pond  that  preceded  the  bog. 

In  the  sea,  the  less  complete  decomposition  of  plants 
and  their  different  composition  result  in  the  deposition  of 
carbonaceous  shales.  A large  deposit  of  this  sort  in  the 
Ohio  valley  represents  an  old  Sargasso  sea.  It  is  very 
rich  in  coal-oil  products  and  was  quarried  for  distillation, 
before  the  introduction  of  bored  wells. 


OUTLINES  OF  NATURAL  HISTORY  4.  1 8 

Animals  leave  deposits  of  mineral  matters  taken  up  by 
them  during  life,  especially  carbonate  and  phosphate  of 
lime,  and  silica.  Phosphorus,  though  existing  in  such 
small  quantity,  is  essential  to  the  animal  organism  : it 
enters  into  the  constitution  of  bone,  and  is  intimately 
connected  with  the  working  of  the  nervous  system  ; it  is 
secreted  especially  by  the  King  Crab  {Limuhis)  and  the 
Brachiopods.  Carbonate  of  lime  is  secreted  by  many 
forms.  It  is  deposited  on  the  floor  of  the  deep  sea  chief- 
ly by  the  tiny  Foraminifera  which  live  at  the  surface 
and  whose  dead  shells  and  tests  are  constantly  raining 
down  upon  the  bottom  to  add  to  the  slowly  growing 
deposit.  But  shallow  water  deposits  are  due  to  fixed 
animals,  and  especially  to  corals. 

Rising  from  the  sea-floor  in  the  So.  Pacific  O.  are  many 
mountain-like  masses  with  steep  slopes,  coming  above 
the  surface  as  ridges  enclosing  shallow  basins  of  water. 
These  are  solid  masses  of  carbonate  of  lime  in  the  forms 
of  coral  skeletons,  with  living  corals  at  the  top,  and  are 
called  coral  islands  or  atolls.  The  sea  is  constantly 
wearing  away  these  masses  by  its  beating  on  the  outside, 
while  the  basin  or  lagoon  within  is  quiet,  though  con- 
nected with  the  ocean  by  one  or  more  openings,  usually 
on  the  leeward  side.  On  the  ridge  above  water  is  found 
a beach  of  white  calcareous  sand,  and  above  a wind- 
piled  heap  of  this  sand  covered  with  a vegetation  chiefly 
of  palms.  On  the  outside  of  the  reef  where  the  sea  beats 
strongest  are  found  the  dome-shaped  corals  which  can 
withstand  the  waves  (Maeandrina,  Astraea,  etc.)  ; in 
more  protected  situations  are  the  branched  forms  (Madre- 
pores), while  in  the  inner  basins  are  many  very  delicate 
corals  and  other  animals.  The  crevices  and  tide  pools 
everywhere  swarm  with  various  forms  of  animal  life 
suited  to  them. 

Along  the  N.  E.  coast  of  Australia  is  a great  wall  of 
coral  over  looo  mi.  long  and  several  thousand  feet  high, 
known  as  the  Australian  barrier  reef. 


DYNAMICAL  GEOLOGY. 


19 


These  coral  reefs  and  islands  depend  on  a sufficiently 
rapid  growth  to  counteract  and  to  gain  on  the  wearing 
action  of  the  sea.  Their  existence  depends  on  the  possi- 
bility of  the  association  of  numbers  of  individuals  in 
colonies  and  on  the  presence  of  currents  bearing  suffi- 
cient food  and  warmth  (70 — 75"^  F.)  to  support  these  col- 
onies in  their  growth  and  to  supply  the  immense  energy 
expended  by  them. 

Atolls  were  at  first  believed  to  be  formed  by  masses  of 
coral  growing  on  sunken  craters  of  old  volcanos,  since 
corals  cannot  live  at  a depth  greater  than  100  feet.  But 
dredging  showed  the  whole  mass  of  the  cones  to  be  com- 
posed of  coral,  and  Mr.  Darwin  began  to  collect  data 
concerning  them.  He  found  that,  while  most  of  the 
atolls  enclose  open  lagoons,  some  of  them  have  islands 
of  volcanic  rock  in  the  centre,  and  in  other  cases  the 
island  is  a mass  of  rock  closely  fringed  by  coral,  without 
water  between. 

From  these  facts  he  deduced  the  now  accepted  theory 
of  their  origin,  that  they  are  coral-reefs  which  started 
about  the  sides  of  volcanos  when  they  became  extinct. 
As  the  sea  floor  slowly  sunk  carrying  down  the  volcano, 
the  corals  grew  upward  until  at  last  in  many  cases  they 
alone  remain  to  show  the  site  of  the  old  volcano,  while 
in  other  cases  its  top  remains  as  the  island  in  the  lagoon. 
This  would  indicate  a slow  and  steady  subsidence  of  the 
sea-floor,  as  a rapid  one  would  quickly  have  carried  the 
corals  beyond  their  depth  ; and  we  have  a good  illustra- 
tion of  the  normal  subsidence  of  the  sea-floor  caused  by 
the  cooling  of  the  earth’s  interior,  and  resulting  in  a de- 
crease in  the  earth’s  diameter  of  one  to  three  feet  in  one 
thousand  years. 

From  old  coral-reefs  we  can  determine  the  set  of  ancient 
currents  and  from  the  latter,  the  conformation  of  land 
masses  ; so  that  fossil  coral-reefs  give  us  valuable  aid  in 
plotting  the  early  geography  of  the  earth. 


OUTLINES  OF  NATURAL  HISTORY  4. 


20 


Cohesive,  Capillary  and  Gravitative  Attraction. 

Cohesion  and  Crystallization.  Dana^j>.  62^, 

Cohesion  is  very  variable  in  rocks,  and  each  rock  mass 
has  two  elements  of  cohesion,  that  of  the  whole  mass, 
and  that  of  the  units  constituting  the  mass.  But  in  a 
given  rock,  the  cohesion  is  constant,  though  sometimes 
increased  or  decreased  by  the  effect  of  crystallizing  action. 
Crystallization  acts  only  till  its  work  is  accomplished,  a 
comparatively  short  time. 

Some  rock  are  very  elastic  and  this  quality  usually  in- 
creases with  density.  A species  of  rock  known  as  Ita 
columite  and  resembling  sandstone,  is  frequently  asso- 
ciated with  gold-bearing  rock.  It  is  so  ffexible  that  the 
ends  of  a strip  supported  at  the  middle  will  drop,  of  their 
own  weight,  to  a certain  point,  beyond  which,  however, 
they  are  rigid. 

All  rocks  contract  and  expand  under  cold  and  heat,  the 
amount  varying  for  different  rocks.  This  has  to  be  taken 
into  account  in  the  construction  of  fortresses  and  is  of 
much  importance  in  Geology,  when  we  consider  the  great 
extremes  of  temperature  through  which  many  rocks  have 
passed. 

Capillarity.  Dana^f.  62g. 

This  has  an  important  effect.  On  a beach  the  waves 
grind  up  the  sand  and  pebbles  much  less  than  they  would, 
were  it  not  for  the  thin  layers  of  water  between  the  grains, 
which  are  held  there  by  capillary  action.  During  long 
and  severe  droughts,  on  salt  marshes  which  have  been 
reclaimed  and  brought  under  cultivation,  there  may  be 
seen  efflorescences  of  salt,  brought  up  from  considerable 
depths  by  capillary  action.  Roofing  slates  are  artificially 
separated  by  pouring  water  upon  the  edge  of  o block,  when 
it  is  drawn  into  the  cleavage  planes,  which  were  before 
invisible. 


mm 


DYNAMICAL  GEOLOGY. 


21 


Gravitation.  Dana^  p.  6jo. 

This  force  appears  in  some  way  in  nearly  all  motion  on 
the  earth’s  surface,  though  always  in  conjunction  with 
other  forces.  The  soil  on  sloping  hillsides  tends  con- 
stantly to  slip  down.  But  the  chief  effect  of  gravity  is  to 
pull  down  rocks  from  cliffs  or  elevations.  The  “Diab- 
lerets”  is  an  amphitheatre  near  the  upper  Rhone,  on  top  of 
whose  high  vertical  walls  formerly  stood  five  mountain 
peaks.  The  surface  on  which  they  rest  is  soft  and  slopes 
inward  toward  the  amphitheatre.  Two  of  these  peaks 
have  already  leaped  off  and  another  will  soon  do  so,  having 
slipped  down  the  slope  to  the  edge.  Several  destructive 
falls  of  this  sort  have  taken  place  in  the  Alps.  Earth- 
quakes greatly  assist  in  starting  an  action  which  gravi- 
tation can  continue  and  complete.  An  important  work 
of  gravity  is  the  breaking  up  of  rock  masses  and  so 
assisting  in  soil-formation. 

The  Atmosphere.  Dana^  p.  630. 

The  atmosphere  is  interesting,  because  it  is  the  mechan- 
ism through  which  celestial  energy  is  applied  to  the 
earth’s  surface.  All  life  is  absolutely  dependent  on  the 
atmosphere,  aquatic  forms  depending  on  the  air  contain- 
ed in  the  water.  In  keeping  the  water  aerated,  storms, 
which  stir  it  up  and  cause  breakers,  are  very  effective. 

The  atmosphere  is  to  be  considered  (i)  as  a reservoir 
and  (2)  asin  motion.  It  is  a reservoir  of  oxygen,  hydrogen 
and  CO2  . Oxygen  is  taken  from  the  air  by  all  living 
things  and  by  the  oxidation  of  various  substances  ; a 
small  amount  is  returned  by  the  reduction  of  oxides,  but 
how  the  great  bulk  gets  back  is  not  clear.  CO^  is  taken 
by  all  plants  and  is  returned  by  decay  and  combustion. 

The  atmosphere  in  motion  is  the  wind.  Its  greatest 
authenticated  velocity  is  about  100  miles  per  hour,  though 
its  force  much  exceeds  this  in  volcanic  explosions  or  rock- 
falls.  The  light  weight  of  the  air  makes  its  rending 
power  very  small ; but  on  soft  rocks,  sand,  driven  by  the 


OUTLINES  OF  NATURAL  HISTORY  4. 


22 


wind,  often  has  considerable  cutting  power.  Loose  sand 
not  held  in  place  by  vegetation,  as  on  sea-shores  and  de- 
serts, is  often  heaped  into  hillocks  or  "dunes.”  These  start 
like  ripple-marks  <?. ) and  often  reach  a considerable 

size.  The  wind  is  constantly  being  blown  over  the  crest 
from  the  windward  side  of  these  dunes  and  so  they  grad- 
ually travel  across  country  with  the  prevailing  winds, 
sometimes  overwhelming  villages.  In  this  way  deserts 
often  extend  their  limits  and  so  the  Sahara  has  encroach- 
ed upon  ancient  Egypt,  coming  down  in  places  nearly  to 
the  Nile. 

Fine  sand  is  sometimes  carried  long  distances  by  the 
wind  and  finally  deposited,  the  successive  deposits  form- 
ing, at  length,  a thick  layer  over  the  surface,  as  in  the 
valley  of  the  Hoangho  in  China.  These,  which  have 
been  but  recently  studied,  are  called  "Loess”  deposits. 

Note. — In  the  study  of  Geology  we  must  radically 
correct  our  preconceived  notions  of  time  and  its  duration. 
Our  conceptions  are  based  on  successions  of  events  and 
their  impressions  on  the  mind ; but  we  must  learn  to 
appreciate  the  stability  of  inorganic  forms  and  pheno- 
mena as  compared  with  the  momentariness  of  organic 
life,  and  that  geologic  time  is  not  to  be  measured  by 
human  life  or  by  the  duration  of  all  life. 

In  considering  geologic  time  we  must  give  up,  if  pos- 
sible, our  ideas  of  a beginning  and  an  end,  in  order  to 
appreciate  its  vastness. 

Water.  DanaN'  ^35- 

Water  does  a much  greater  work  than  the  air.  Its  func- 
tions are  of  two  sorts,  that  of  storage  and  that  of 
afflying  energy.  The  former  includes  its  carrying 
power  for  heat  and  various  substances ; the  latter  in- 
cludes its  action  in  falling  and  is  due  to  heat  and  gravity. 

Substances  carried  by  water  may  be  in  perfect  solu- 
tion, so  as  not  to  be  affected  by  gravity,  or  they  may  be 
mechanically  suspended  so  that  gravity  constantly  tends 
to  make  them  settle  to  the  bottom. 


DYNAMICAL  GEOLOGY. 


23 


Heat  causes  water  to  evaporate  and  rise  in  vapor  to 
form  clouds,  in  which  form  it  has  great  energy  of  posi- 
tion and  tends  to  fall  in  obedience  to  gravity.  When  it 
falls,  it  is  as  rain  or  snow,  and  the  roles  played  by  these 
two  forms  of  water  are  very  different.  Rain  which  has 
fallen  is  either  reevaporated  and  reprecipated  or  else  it 
passes  into  the  ocean  through  streams,  and  is  there 
evaporated.  By  the  rainfall  of  a country  may  be  meant 
simply  the  number  of  times  its  water  is  evaporated  and 
precipitated.  Forests  act  as  sponges  to  hold  water 
which  has  fallen  until  it  is  again  vaporized  and  so  increase 
rainfall,  but  probably  do  not  increase  the  amount  of 
rain  from  the  ocean  on  a region.  Thus  the  complete 
circuit  of  water  is  from  the  clouds  to  the  earth  as 
rain,  through  the  rivers  to  the  ocean  and  thence  back  to 
clouds. 

Water  at  high  speed  has  great  energy,  whether  in 
waves  or  current.  The  maximum  speed  of  currents  is 
about  10-12  miles  per  hour,  and  their  carrying  power  in- 
creases as  the  fourth  power  of  the  speed. 

Water  above  the  sea  level  has  energy  of  position 
which  it  tends  to  expend  by  running  down  to  sea, 
growing  less  and  less  powerful  as  it  approaches  the  sea 
level. 

Rivers.  Dana^  'p-p,  6 26-66 j. 

Water,  falling  as  rain,  partly  penetrates  the  soil,  as  ground 
water,  and  partly  remains  as  surface  water.  Of  all  this 
water  streams  carry  away  from  15%  to  25%,  the  rest 
being  evaporated.  A basin  of  water,  the  evaporation  from 
which  exceeds  the  supply,  becomes  a salt  lake  or  dead 
sea.  The  ocean  is  prevented  from  becoming  so  salt 
and  alkaline  by  the  action  of  its  organic  life. 

The  effect  of  a stream,  {p.  6^1)  as  soon  as  it  becomes 
permanent,  is  to  channel  out  a bed  and  to  cut  and  leach 
away  its  sides.  By  its  action  the  surface  of  its  valley 
is  lowered  about  one  foot  in  50CO  years  on  an  average. 


OUTLINES  OF  NATURAL  HISTORY  4. 


24 


A stream  always  tends  to  increase  its  curves  by  building 
on  to  one  bank  and  cutting  away  the  other  until,  at 
last,  a bow  is  formed,  with  a narrow  neck ; then  the 
stream  cuts  through  the  neck  and  leaves  its  old  bed 
around  the  bow.  But  a change  like  this  necessitates  the 
reconstruction  of  all  the  curves  in  the  river  below,  since 
all  the  curves  of  a stream  have  a definite  relation.  Thus 
a stream  slowly  works  over  its  valley,  and  successive 
maps  of  a system  at  different  stages  of  its  history  would 
show  very  various  courses,  though  each  would  have  a 
certain  relation  to  the  others.  The  ratio  of  the  leaching 
action  of  a stream  to  its  downward  cutting  depends 
largely  on  the  character  of  the  bed ; in  the  canon  of  the 
Colorado  (_^.  the  leaching  action  is  almost  noth- 

ing, from  the  hardness  of  the  rock,  and  the  river  has  cut 
deep  gorges,  with  nearly  vertical  sides. 

The  topographic  effects  of  erosion  55/)  are  influ- 
enced by  three  factors  : the  hardness  of  the  rock,  the 
structure  of  the  rock  as  to  jointing,  etc.,  and  the  attitude 
of  the  beds.  The  second  of  these  determines  the  amount 
of  rending  of  the  rocks  by  the  penetration  and  freezing  of 
water.  The  third  also  often  has  much  influence  on  the 
courses  of  streams,  where  there  is  a tilting  of  the  beds. 
In  such  a case,  where  there  is  a soft  stream  lying  be- 
tween hard  ones,  the  former  is  worn  away  by  the  water, 
and  thus  the  hard  layers  above  are  undermined,  and  at 
last  cave  in,  so  that  the  bed  of  the  stream  gradually 
works  away  in  the  direction  of  the  dip  of  the  strata. 
Where  water  flows  over  folded  strata,  its  cutting  action 
is  greatest  on  the  ridge  and  least  on  the  depressions. 
This  is  due  to  the  looser  and  more  open  character  of  the 
rock  on  an  anticlinal,  and  its  compacted  and  solid  char- 
acter in  a synclinal.  These  variations  in  hardness  and 
dip  of  strata  thus  can  produce  infinite  variations  in  de- 
tails of  mountains  and  rivers,  and  are  of  great  influence 
in  determining  the  relief  of  a region.  The  above  con- 


DYNAMICAL  GEOLOGY. 


25 


siderations  may  give  us  an  idea  of  the  wonderful  elas- 
ticity in  the  course  of  a river,  and  show  that  the  position 
of  a stream  at  a given  time  is  only  temporary  and  is  de- 
rived from  all  past  conditions. 

The  pot-holes  in  the  New  England  Mts.  (^.  55J-) 
are  connected  with  glacial  action.  When  the  ice-sheet 
was  retreating,  near  the  end  of  the  glacial  period,  it  acted 
in  many  places  as  a dam  to  block  up  river-valleys  and 
form  great  ponds  which  rose  until  they  overflowed  their 
water-sheds  and  poured  over  into  the  opposite  valleys. 
The  rush  of  these  torrents  over  the  ridges,  during  the 
time  when  their  outlets  were  dammed  by  ice,  was  the 
cause  of  the  formation  of  these  pot-holes. 

The  transporting  effects  of  rivers  {p- SSJ-)  vary  much 
with  the  characters  of  their  valleys  and  other  conditions, 
the  long  streams  carrying  out  much  more  material  in 
proportion  than  the  smaller  ones.  It  was  formerly 
believed  that  a large  part  of  the  sediments  of  rivers  are 
carried  far  out  to  sea,  but  it  is  now  known  that  sea-water 
has  the  peculiar  effect  of  causing  the  precipitation  of 
substances  at  a very  rapid  rate,  so  that  little  sediment 
is  carried  far  beyond  the  mouths  of  streams.  Substances 
not  merely  in  suspension,  but  truly  dissolved  in  water 
may  be  carried  any  distance.  This  deposition  of  sedi- 
ments is  important  in  enabling  us  to  make  out  old  river 
deltas,  from  the  structure  of  the  strata,  which  always 
show  considerable  woody  matter  and  also  cross  bedding 
of  layers  of  sediments.  From  the  study  of  old  deltas  in 
connection  with  present  geography,  much  can  be  learned 
of  the  rivers  which  formed  them.  Thus  Gay  Head, 
Mass,  is  part  of  the  delta  of  a large  river  which  once 
drained  New  England,  and  perhaps  west  to  the  Hudson 
River  ; and  the  Isle  of  Wight  is  formed  by  delta-deposits 
of  the  old  Rhine,  when  Great  Britain  was  a part  of  the 
continent  of  Europe. 


